Blog / Go Language Spec

The type of a function call expression

December 13, 2023

Calls

… The type of the expression is the result type of F. A method invocation is similar but the method itself is specified as a selector upon a value of the receiver type for the method.

math.Atan2(x, y)  // function call
var pt *Point
pt.Scale(3.5)     // method call with receiver pt

We recently talked about method calls already, so we don’t need to expand much more on those.

The main thing I want to discuss here is the first sentence: “The type of the expression is the result type of F.”

This is probably intuitive, and we actually demonstrated it yesterday, but it’s worth calling out explicitly, I think.

Given a function type of func (int, int) int, the type of the expression is the result type int.

So what about a function type of func() (int, error)? What is the type of the expression calling it? Well, it’s (int, error). That is, the expression resolves to multiple values, each of a distinct type (int and error respectively).

And this is where the rule we discussed yesterday comes in, that (except for a mystery exception not yet discussed) you can only use a function call as an argument to a function, when it returns a single value only.

Quotes from The Go Programming Language Specification Version of August 2, 2023

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Refresher on methods

In yesterday’s email, I failed to include the example included in the spec, which doesn’t make sense to include otherwise. I’ve updated the web version of the email in case you’re super curious to see what I left out. We’ve already covered these details, but they’re logically mentioned again in the section about function calls, so we’ll cover them here: Calls … A method call x.m() is valid if the method set of (the type of) x contains m and the argument list can be assigned to the parameter list of m.

Special case: Multi-value arguments to functions

You may recall that when we started the section on function calls, we were told about a “special case”… well here it is! Calls … As a special case, if the return values of a function or method g are equal in number and individually assignable to the parameters of another function or method f, then the call f(g(parameters_of_g)) will invoke f after binding the return values of g to the parameters of f in order.

nil functions

Calls … Calling a nil function value causes a run-time panic. So, calling a nil function causes a panic. Sensible enough. But when would you ever run across a nil function? Let’s look at some examples. This is probably the most obvious example, and perhaps one that popped to your mind: var fn func(int) // fn is of type func(int), but uninitialized, so nil fn(3) // run-time panic But let’s consider another example, which can be a bit more confusing.